Mimick `.await` completions as snippets

[DoctorKrolic]

.await and .awaitfare really useful completions. However, in such form they behave a lot more like snippets rather than just a keyword completions. So I made tiny visual changes to them:

1. Change icon to snippet when in a postfix form
2. Do not add 'await' keyword to the tooltip of await completion when in postfix form

Looks like this:

I think it makes more sense to display them this way. As a user I expect keyword completions to just insert a keyword and maybe do some slight change, e.g. add an async to the declaration. But not flip the whole expression.